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donít just clip wings to make it go faster. There is a reason for
it," Bruce Boland says in response to a sophomore question about
gaining speed out of a racing airplane. "You can hurt your speed
potential if you cut too much wing off."
During a 1995 interview, Boland spent the better part of four hours with this author explaining how to make a racer go faster, why certain things work, and why most of them donít. Other interviews with Pete Law, Tiger, Bill Rheinschild, Darryl Greenamyer and Bill Kerchenfaut filled in the shady places where speed hides. It is half physics and half black art. Modifying an airplane to go faster at Reno is never an easy task, nor is it cheap. Looking back at the history of air racing and record setting aircraft, certain trends become apparent.
You Gotta Have Horsepower
For the purpose of this article, weíre going to assume we have a P-51 Mustang. Our goal is to modify it to a gold level Reno racing plane. In terms of airframe, engine and performance the stocker is a know quantity. It will do about 370 mph at 61 inches of manifold pressure and 3100 rpm at Reno.
Not good enough... Where do we start?
Horsepower - and lots of it. I donít care if you fly a barn door - if you put enough power behind it, it will go faster. Our airplane is considerable more refined than a barn door, so weíre in good shape.
The physics behind a higher horsepower requirement lie in drag. In simple terms, as speed increases, drag increases as the square. So, we have double the drag at 400 mph as we do at 200 mph. As we go faster, drag continues to build at twice the rate. If you think about it, the difference between a 450 mph racer and a 490 mph racer are vast. They simply arenít in the same league.
What is the major difference? Power. We can compare two Mustangs; Bill Rheinschildís Risky Business and Terry Blandís Dago Red. Risky Business has flown its fastest laps around 450 mph, and itís pretty maxed out there. Itís generally a 430 mph racer; the engine just doesnít make the power to reach the next regime. Although it has all of the race systems, a slick wing and so on, it just canít power through the drag rise.
On the other hand, Dago Red can. Itís Merlin is set up to run up to 145 inches of manifold pressure and up to 3,700 rpm. Iím not saying that is where it is run, but itís capable of it. At 105 inches and 3,400 rpm, Dago is a solid 490 airplane. This is mainly because of the horsepower.
Todayís racing Merlins are not your typical factory units. Builders like Dwight Thorn and Mike Barrow reinforce the cases, use stronger Allison connecting rods, and special high-compression pistons to obtain the 3,800 horsepower it takes to get a Mustang to go 490 mph. And that isnít even the beginning of it. Their secrets involve increased oiling, blower gear ratios, prop gear ratios, aftercoolers, tubes and ADI. Weíre also getting into anti-friction coatings within the Merlin as a crossover technology from NASCAR. Jack Hovey has been building Merlins with roller cams in an attempt to reduce internal friction loss. This is an attempt to return the horsepower it takes to drive the cams back to the crankshaft. Unfortunately, all of the this slick stuff is being done to 60 year old engines.
Having a race modified engine begins a domino effect with the airplane. Our Merlin is going to turn faster, so each accessory unit coming off the engine is also going to turn faster; generator or alternator, oil pump(s), mag drive, the prop, and the supercharger. This last item introduces another problem.
Since we are turning the engine and supercharger faster, we are increasing the manifold pressure from a stock 61 inches to anywhere up to 150 inches. That means more than double the compression of intake air, and the resulting rise in its temperature. We can not have this - any induction air temperature over 100 degrees C will cause the engine to detonate and fail.
We get around this by cooling the induction air with a mixture of water and alcohol called ADI. Anti Detonation Injection is sprayed into the intake downstream from the supercharger and cools/densifies the air. In effect, it allows the engine to live at high power settings. An ADI pump failure, running out of fluid or forgetting to turn the system on will result in major problems or a blown engine.
Itís Still Hot
Now that the intake air is happy, we have other heat problems to deal with. Our 3,800 hp Merlin makes its power by producing heat. Since the engine is liquid cooled, we have superheated coolant routed to a radiator or a heat exchanger. Just like a carís radiator, the air flows through it and carries away excess heat. The problem is, weíre producing so much power and heat, that this setup will not work.
Our answer again lies with spraying water, this time over the face of the heat exchanger. Itís actually the same fluid as ADI, because the cooling properties of water and alcohol far exceed those of air. Dago Red, Strega and other Mustang racers have these spray bars - and youíll notice a hearty trail of water vapor trailing the coolant door during a race. The air entering the scoop simply becomes a means to get the water to the face of the heat exchanger, where is flashes to steam, and carries the heat out the exit door. Most non-racers find it interesting to learn a Mustang racer goes through about 80 to 110 gallons of water during a race!
The rest of the engine modifications include, but are not limited to, hard mounting to the engine bearer, special magnetos or electronic ignition, lower propeller reduction gears, spark plugs, Merlin Fingers, roller cams, additional oiling, case strengthening, nitrous injection and larger exhaust stacks.
Making the Air Happy
All of our horsepower is great, but to realize our 490 mph speed goal weíre going to have to make some aerodynamic changes. Most people know that racing Mustangs have clipped wings, cut down canopies and fancy wingtips. But the real changes are much deeper than that.
The most obvious change is a clipped wing. The reason this is done stems from the weight reduction of the Mustang from its wartime configuration. These airplanes have no armor, no heavy radios, no bomb racks and no machine guns. Compared to wartime Mustangs, the racers are extremely light. Their gross weight does not require the wing area that a heavier airframe would. The answer? Remove some wing... Doing so not only reduces induced drag, it also preserves the proper straight and level pitch attitude at speed. The tradeoff is reduced aileron span, higher approach and glide speeds, and reduced operating altitude.
The same principle applies to the horizontal tail; the airplane is lighter and does not require as much tail down force. Some racing Mustangs have the end cap removed, while others retain it.
Now we have a wing with the proper area, but we have to work on it to make it efficient. North American made the wing with a laminar flow airfoil, but that doesnít mean it came from the factory with those exact production tolerances. Even if it did, it sure isnít in the same shape today. Weíre going to have to clean up the panel lines, install the tanks for the spray bar water and ADI, then contour the wing to a true laminar shape. Glassing the wings allows the profile to be built up in certain areas, sanded down in others and generally perfected.
Weíve already touched on the radiator scoop, but that dealt with the engine. We have to deal with aerodynamics now. Cutting drag on the airframe is paramount to getting a 490 mph lap speed. North American did a splendid job of designing their scoop for the Mustang. They even say it produces thrust at certain speeds and altitudes. But it also takes a huge chuck of air through the airframe; that means drag.
The racers have cut down the deepness, profile and intake area of their scoops. There have been several iterations of this theme. Bruce Boland designed them with a splitter to get the intake face out of the turbulent boundary layer on the bottom of the wing. Others, like Strega, have no lip and allow airflow from the bottom of the wing to enter the scoop. Whatever the case, the amount of air a racer takes in is dramatically reduced. That means a reduction in cooling drag.
The exit ramp of the scoop has also received attention. The stock Mustang exit door is rather short, and fails to harness the low pressure exit air in an efficient manner. Racers have found that extending the door aft controls the airflow from the scoop and nets them an amazing 11 mph. In racing, that is a giant leap in speed.
Some of the deeper modifications center on details inside the airframe. The flaps are reflexed, air holes are plugged, lighter components are used, and the center of gravity is moved aft. This makes for a faster and more efficient racer, but the flying qualities change so much that some pilots donít like it very much. In many cases, these racing Mustangs take forward stick when making pylon turns!
There are hundreds, if not thousands of other changes that can be made. The vertical tail is normally offset to reduce the torque and P-factor effects. On a racer, it is zeroed out to reduce drag at race speeds. The incidence of the horizontal tail may also be changeds to lessen trim drag at race speeds. It is also rumored that one champion racing Mustang had its wing incidence changed; a major undertaking. To finish our racer, weíll install cut down canopies, fairings and special wing tips contribute to lower drag and a stylish look.
Itís Not Easy
These are the basics of racing modifications. If weíre lucky, somebody like Jim Larsen or Pete Law will write a book that details the refinements made to these racers, but also the physics and test data that backs it all up. Many are tried and true, while others are subject to debate and conjecture.
What we have not touched upon here is the rest of the equation. You might have all the right modifications to go fast, but you have to have the rest of the package in place, too. The owner, the crew chief, the pilot, the crew and the support staff all have to fall into place and work as a cohesive unit. That just gets you into the game. Once there, the field is leveled with luck.
Story by Scott Germain - WarbirdAeroPress.com. Copyright 2005. All Rights Reserved.
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